During the past year, we have used synchrotron radiation at CHESS to determine the structures of two new protein DNA complexes in the Cre-loxP site-specific recombination pathway and the structure of a thermophilic arginine repressor in the apo form. In the Cre-loxP project, we have focused on the structure of Cre bound to a symmetric, fully mobile Holliday junction intermediate using a cleavage-deficient recombinase mutant. This structure has been refined to 2.1 Angstroms resolution. We have also begun to investigate the structure of the recombinase bound to duplex substrates prior to strand cleavage. One such structure has been determined using CHESS data and is currently being refined with data that also extends to 2.1 Angstroms. Using both a 2.5 Angstrom native data set and a selenomethionine derivative data set measured at CHESS, we have recently determined the structure of the 100 kD hexameric arginine repressor.